Liners aren't essential but are very convenient. Liners let the moisture through but catch the solids for easier disposal.
Littlelamb paper liners are convenient but do cost money over time. They are made from recycled paper (with no polyester content so they will break down and won't end up on a beach somewhere) biodegradable, flushable (one at a time), live septic tank friendly, compostable and chlorine bleach hasn't been used in the manufacture. However, do not flush liners if you have old, broken or damaged pipes. All the kits now come with an equal amount of washable fleece liners as nappies.
Paper liners, whilst effective, cost money over time. On 6500 nappy changes the cost of paper liners work out at £130 and that's if they are purchased locally and you don't have to add the cost of delivery.
Littlelamb fleece liners are made from the softest polyester fleece and they are over locked on the seams.
Which ones to use then?
As a rule of thumb use washable fleece liners when you're around the home and paper liners when you're out and about.